text: """Derivation of variable `lwcre`."""
  
  from iris import Constraint
  
  from ._baseclass import DerivedVariableBase
  
  
  class DerivedVariable(DerivedVariableBase):
      """Derivation of variable `lwcre`."""
  
      @staticmethod
      def required(project):
          """Declare the variables needed for derivation."""
          required = [
              {
                  'short_name': 'rlut'
              },
              {
                  'short_name': 'rlutcs'
              },
          ]
          return required
  
      @staticmethod
      def calculate(cubes):
          """Compute longwave cloud radiative effect."""
          rlut_cube = cubes.extract_strict(
              Constraint(name='toa_outgoing_longwave_flux'))
          rlutcs_cube = cubes.extract_strict(
              Constraint(name='toa_outgoing_longwave_flux_assuming_clear_sky'))
  
          lwcre_cube = rlutcs_cube - rlut_cube
          lwcre_cube.units = rlut_cube.units
  
          return lwcre_cube
  